1. Field of the Invention
The present invention relates to a fixing device to fix images on recording media, and an image forming apparatus employing the fixing device.
2. Discussion of the Background
In image forming apparatuses, such as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having several of the foregoing functions, fixing devices that fix a toner image on a recording medium such as a paper sheet by heating and pressing the toner image onto the sheet are widely used.
With reference to FIG. 19, configuration of such a fixing device A is described below. As shown in FIG. 19, the fixing device A includes a rotatable fixing roller 100, a heater 200 that heats the fixing roller 100, and a pressure roller 300 that presses against the fixing roller 100. The fixing roller 100 serves as a rotary fixing member, the heater 200 serves as a heating generator, and the pressure roller 300 serves as a rotary pressure member.
A toner image T on a sheet P is fixed by heating and passing the sheet P on which an unfixed toner image T is formed through a fixing nip N103 formed between the fixing roller 100 and the pressure roller 300.
However, before a fixing process to fix the image on the sheet P is performed, a transfer process to transfer the image onto the sheet P must be executed.
An intermediate-transfer type image forming apparatus like that shown in FIG. 19 includes, as a transfer mechanism B transferring the image onto the sheet P, for example, an intermediate transfer belt 400 that carries images, a transfer roller 500, and a facing roller 600.
The transfer roller 500 presses against the facing roller 600 through the intermediate transfer belt 400.
When the image is transferred onto the sheet P, the sheet P is passed through a transfer nip N102 formed between the transfer roller 500 and the facing roller 600, timed to coincide with the rotation of the image on the intermediate transfer belt 400, and then, the image on the intermediate transfer belt 400 is transferred onto the sheet P at the transfer nip N102.
As can be seen from FIG. 19, as the sheet P passes through the transfer nip N102, the sheet P curves along the outer circumferential surface of the facing roller 600. Therefore, depending on the material of the sheet P, after the sheet P is passed through the transfer nip N102, the sheet P is still curved and transported onward in that curved state. The problem is that thereafter, when the curved sheet P is transported to the fixing device A, the sheet P might jam and wrinkle because the sheet P does not enter the fixing nip N103 properly.
To solve this problem, as shown in FIG. 19, two guide members 700 and 800 are located upstream from the fixing nip N103 in a direction in which the sheet P is transported, and these guide members 700 and 800 guide a leading-edge of the sheet P from the transfer nip N102 to the fixing nip N103.
However, when the guide member 700 is located on the side of an unfixed-image surface, that is, a surface of the sheet P on which the unfixed toner image is formed (left side surface in FIG. 19), the guide member 700 contacts the unfixed-image surface, disturbing the image T on the sheet P and thus degrading image quality.
In an effort to avoid this problem, several approaches have been proposed to avoid having the guide member contact the unfixed-image surface of the sheet P.
For example, one known image forming apparatus includes a spur that is rotatably attached to the guide member disposed on the side of the unfixed-image surface of the sheet P. Multiple projections are continuously formed on an outer circumferential surface of the spur. In this case, only a small point of the spur contacts the surface of the transported sheet P, and the sheet P can be prevented from contacting the guide members. Therefore, contact of the unfixed image surface of the sheet P with the guide members and consequent image quality degradation can be prevented.
However, as shown in FIG. 20, when the curved sheet P is transported to the fixing nip N103, due to the rigidity of the curved sheet P, a trailing-edge portion of the sheet P may be curled to such an extent that a portion of the unfixed toner in the trailing-edge portion does contact the guide member 700. In this case, there are concerns that the image on the trailing-edge portion of the sheet P might be disturbed and the image quality might be degraded.
An additional consideration is that, at present, to make the image forming apparatus compact, small-radius rollers are often used for the transfer roller and the facing roller, and the sheet transport pathway itself is often deliberately curved. As a result, situations in which the sheet P transported to the fixing device is curved can increase. Moreover, the known fixing devices do not include a specific mechanism, to prevent the trailing-edge of the curved sheet P from contacting the guide member.
In view of the foregoing, there is market demand for a fixing device capable of preventing the trailing-edge of the sheet P from contacting the guide member, thereby preventing degradation of image quality.